Decoding lactate signals in Alzheimer's disease: redox control, receptor pharmacology, lactylation chemistry, and neuroglial vulnerability.

Alzheimer's disease (AD) develops within a metabolically heterogeneous brain in which lactate functions as an oxidative substrate, a redox-coupled metabolite, a proton-linked transport signal, a receptor ligand, and a precursor of lysine lactylation. These roles are often considered independently, obscuring why lactate supports neuronal function in some settings yet accompanies persistent inflammation and neurodegeneration in others. This review introduces a lactate signal-decoding framework that emphasizes cellular interpretation rather than concentration alone. The framework integrates the lactate/pyruvate ratio, the cytosolic reduced-to-oxidized nicotinamide adenine dinucleotide (NADH/NAD + ) state, lactate dehydrogenase (LDH) isoenzyme context, proton-coupled monocarboxylate transport, extracellular pH, hydroxycarboxylic acid receptor 1 (HCAR1) signaling, and enzymatic or non-enzymatic lactylation. We compare neuronal, astrocytic, microglial, and neurovascular responses and examine how aging, apolipoprotein E ε4 (APOE4), amyloid pathology, hypoperfusion, sleep disruption, and systemic metabolic disease reshape them. Particular attention is given to the chemistry and analytical validation of histone and non-histone lactylation; the proposed interaction of tau lactylation with other post-translational modifications; and links to proteostasis, iron homeostasis, and mitochondrial quality control. As a hypothesis-generating model, AD progression may involve loss of coordination among lactate transport, oxidation, receptor signaling, pH control, and covalent modification. This framework prioritizes restoration of metabolic coordination over indiscriminate lactate suppression and identifies biomarker and experimental requirements for clinical translation.